It is well known that various reproduced pictures are produced from color originals of continuous tone by conducting various tonal conversion of pictures, such as halftone gradational representation (multi-value area gradational representation) as seen in printing, binary-value gradational representation as seen in ink-jet recording, and direct density gradational representation as seen in sublimed pigment thermal transfer recording. There are common problems and demands in the conventional tonal conversion technique of pictures. They will be described hereinafter taking printing work as a representative example in picture reproduction field.
In the electronic plate-making process, color-separation work is carried out by using a color scanner or a total scanner (hereinafter simply called "scanner separation") which is highly advanced in mechatronics. It is the present state of art that no rational tonal conversion theory of pictures has yet been established for conversion of color originals of continuous tone into printed pictures in halftone. Practical color-separation work is heavily dependent on the experiences and perception of operators, while expensive color scanners highly advanced in mechatronics are used therein as working means, as previously described. The same situation can be found where a color original has non-standard picture quality, that is, the density range of a color original is removed from that of a color original having a standard picture quality, for example over- or under-exposed, or a color original is color-fogged. Concerning color-fog in particular, actual color originals are all color-fogged more or less, but no rational technique has yet been established for removing the color-fog. When an operator attempts to remove color-fog through color separation by using a scanner, the operator has to perform the work on a trial and error basis under the circumstances.
A scanner which requires a large investment is thought to actually have a high operation rate in the light of its sophisticated technology. However, its actual operation rate is still on an order as low as about 30% to 40% on average. Consequently, this prevents improvements of the current working conditions that workers are customarily forced to work overtime works many hours, even beyond midnight. Moreover, this also prevents the enhancement of each company's financial status and the competitive power.